JP2003-83707A discloses a strain generating member (sensor board) including a detection hole at a center, and first and second fixing holes at both sides of the detection hole. A shaft-shaped detection member is press-fitted to the detection hole to which an external force is applied. In addition, shaft-shaped first and second fixed members are press-fitted to the first and second fixing holes, respectively, to which a reaction force is applied. First, second, third and fourth strain gauges are provided between the detection hole and the first and second fixing holes. The first to fourth strain gauges are connected to each other by means of a circuit pattern so as to constitute a bridge circuit. The external force received by the detection member is measured on the basis of an output from the bridge circuit.
According to the structure of the aforementioned strain member, each of the shaft-shaped members (i.e., the detection member and the fixed members) is press-fitted to each of the holes (i.e., the detection hole and the fixing holes). The shaft member is press-fitted to the hole over an entire periphery. Thus, an entire inner periphery of each of the holes receives a pressing force from each of the shaft members. As a result, an internal stress is generated at the strain generating member by means of the press-fitting of the shaft members. The internal stress influences a detection accuracy of the strain gauges attached to the strain generating member. The detection accuracy of the strain gauges also influences a detection accuracy of a passenger seated on a seat for a vehicle.
A need thus exists for a load detection device for a vehicle seat which is not susceptible to the drawback mentioned above.